1. Field of the Invention
This invention relates to a four-wheel drive motor vehicle comprising means for effecting an automatic change from two-wheel drive mode to a four-wheel drive mode in dependence on the slip of the two permanently driven wheels.
2. Description of the Prior Art
It is already known to measure the slip of the two permanently driven wheels of a four-wheel driven motor vehicle in that the speeds of said two permanently driven wheels and of the optionally drivable wheels are compared and the four-wheel drive modes is selected in response to a high slip of the permenently driven wheels (FISITA Belgrade, June 2 to 6, 1986, Proceedings, Volume 2, pages 277 to 283). When the four-wheel drive mode has been selected, the optionally drivable wheels must be mechanically disconnected from the power transmission after a short interval of time so that the speeds of the permanently driven wheels and of the optionally drivable wheels can again be compared in order to ascertain whether or not the slip of the permanently driven wheels is still so large that a four-wheel drive is justified. Unless that mechanical disconnection is performed, there will be no speed difference between the permanently driven wheels and the optionally drivable wheels unless the power train leading to the optionally drivable wheels incorporate a differential or a liquid friction coupling. The clutch for connecting and disconnecting the optionally drivable wheels must meet special requirements and a differential will increase the structural expenditure involved and is not justified in some applications.